1. Field of the Invention
The invention relates to a hydraulic control apparatus for an automatic transmission. More particularly, the invention relates to a hydraulic control apparatus for an automatic transmission, which controls hydraulic pressure drained from a friction engaging element.
2. Description of the Related Art
A technology for reducing select shock which occurs during a select operation for changing a position from the D position to the N position has been proposed.
A hydraulic control apparatus for an automatic transmission, which gradually decreases hydraulic pressure supplied to a friction engaging element and reduces select shock is disclosed in FIG. 4 in Japanese Patent Application Publication No. JP-A-07-019326. The hydraulic control apparatus for an automatic transmission disclosed in Japanese Patent Application Publication No. JP-A-07-019326 includes a manual valve which introduces line pressure from a line pressure oil passage to a D range pressure oil passage by a select operation for changing a position to the D position, and which drains the pressure supplied to the D range pressure oil passage through a D range pressure drain oil passage by a select operation for changing the position from the D position to the N position; an electronic hydraulic control valve which adjusts D range pressure from the D range pressure oil passage according to a control command from the outside and then introduces the adjusted pressure to a control pressure oil passage to the friction engaging element; and an orifice which is formed in the D range pressure drain oil passage.
With the hydraulic control apparatus for an automatic transmission disclosed in Japanese Patent Application Publication No. JP-A-07-019326, during the select operation for changing the position from the D position to the N position, the D range pressure can be gradually decreased and the select shock can be reduced by controlling an amount of drained D range pressure using the orifice.
However, in the hydraulic control apparatus disclosed in Japanese Patent Application Publication No. JP-A-07-019326, since the amount of drained D range pressure is controlled by the orifice, a problem is caused. For example, when an oil temperature is low, there is a possibility that the D range pressure is not drained completely, the friction engaging element has a low torque capacity, and the friction engaging element is not disengaged even if a shift lever is at the N position. Also, for example, during the select operation for changing the position from the D position to the R position or from the R position to the D position (i.e, during the garage shift), the friction engaging element needs to be disengaged promptly in order to prevent tie-up (i.e., double lock-up). However, there is a problem that the friction engaging element cannot be disengaged promptly since the amount of drained D range pressure is controlled by the orifice.